1. Field of the Invention
The present invention is related to an antenna structure intended for use with a transmitter located at a great depth below the earth's surface and which includes a metal sleeve mounted on the outer surface of a drill collar which forms part of a drill string used for petroleum exploration.
2. Description of the Prior Art
In drilling carried out in search of petroleum deposits, it is desirable to transmit to the site control cabin at the earth's surface information supplied by sensors located at the bottom of the drilling well which relates to the pressure at the bottom of the well, the density of the mud, temperature or other useful parameters. Thus, there has been developed a system to transmit signals from a transmitter housed in the drill collar of a drill pipe string. For use as a transmitting antenna, one prior art system utilizes a metal element formed by a portion of an actual drill collar, insulated from the drill collar and connected to it by a mechanical connection element made of insulating material.
Thus, for example, in a thesis submitted to the University of Lille in 1969 by Mr. Clarisse, it was proposed to divide the drill-pipe string, at the level at the last drill collar, into two portions which are separated by a bridge made of insulating material and between which an alternating electrical potential difference of specific frequency is established. The lower part of the drill collar then forms part of an antenna and the upper portion thereof is connected to the upper drill-pipes by means of threaded joints and constitutes an axis which radiates and guides toward the surface of the ground an electromagnetic wave capable of remote transmission of signals supplied by telemetering sensors located at the bottom of the drilling well.
A system of this type is also found in an article entitled "Second-Generation MWD Tool" published on Feb. 21, 1983 in the journal entitled Oil & Gas Journal, especially in the last paragraph on page 86 of this publication.
The serious disadvantage of this system is that in order to make an insulating connection between the two metal portions of the drill collar, it is necessary to use a hoop or insulating bridge which reduces the mechanical strength of the drill collar which, as is known, is subjected to considerable torsional and shearing forces.
Another known antenna system includes a metal sleeve made of cut sheet metal or of extremely thick mesh, which is connected in an electrically insulating manner to the drill collar of a drilling string by means of suitable adhesive such as epoxy resin. Thus, the metal sleeve of the antenna is mounted in an insulating manner around the drill collar and is separated therefrom by a layer of bonding material, such as epoxy resin. However, this metal sleeve is subjected to considerable friction at the bottom of a drill hole, particularly from contact with the wall of the well. Thus, it has been found that the sleeve must be made of the same material as the drill collar and must have a sufficient thickness which is close to 1 cm, and at a minimum of approximately 8 mm. The problem with this construction is that the metal sleeve of the antenna is so rigid that it becomes delaminated at opposite axial ends thereof from the drill collar due to strain which results from bending vibrations the drill collar is subject to during drilling. As such, the opposite axial ends of the metal sleeve tend to separate from the adhesive material which causes potential infiltration of the drilling mud between the metal sleeve and the drill collar and in addition causes a loss of electrical insulation.
It is an object of the present invention to provide an antenna structure which retains its resistance to abrasion but yet remains flexible so that it can accommodate the bending stress undergone by the drill collar to which it is attached by a layer of insulating adhesive material.
It is an object of the present invention to provide an antenna structure which includes a metal sleeve forming part of the antenna wherein the opposite axial ends of the sleeve are disposed on the drill collar in such a manner that significant short circuiting of alternating current supplied to the metal sleeve is avoided. To avoid this problem it has been found that the opposite axial ends of the cylindrical metal sleeve forming part of the antenna should be spaced a sufficient distance in the axial direction from the opposite axial ends of the insulating sheath disposed between the cylindrical metal sleeve and the drill collar. Thus, when the cylindrical metal sleeve is mounted in a recess in the outer surface of the drill collar, it has been found that the insulating sheath should extend around 50 cm beyond the opposite axial ends of a cylindrical metal sleeve which may be from 2-6 meters long.
By providing an insulating sheath of sufficient length beyond the opposite ends of the cylindrical metal sleeve, it is possible to minimize the problem of electrical current leakage due to short circuiting through the drilling fluid which circulates in the space between sides of the well and the surface of the drill collar, the drilling fluid characterized by an electrical resistance of generally between 0.3 and 3 ohms/meter. However, since the insulating sheath is necessarily constructed of an insulating material, such as epoxy glue, the exposed portion of the insulating sheath beyond the opposite axial ends of the cylindrical metal sleeve is damaged by abrasion and rubbing against the walls of the well due to its inferior strength and wear resistance compared to the steel drill collar. On the other hand, if the distance that the insulating sheath extends beyond the opposite axial ends of the cylindrical metal sleeve is reduced to several millimeters and if the electrical resistance of the ground across from the cylindrical metal sleeve is relatively high, for example around 100 ohms/meter, nearly all of the current is dissipated in a short circuit with the drill collar, with the result that electromagnetic wave transmission is not possible.